In the oxygen top-blowing steel making process, molten iron, scrap and other starting materials are charged into a converter and then refining of steel is carried out while blowing pure oxygen onto the charge melt through an oxygen lance. At an early or intermediate stage of the blowing, the oxygen vigorously reacts to the melt still containing a substantial level of carbon so that the generanion of carbon monoxide is sufficient to bring about thorough agitation of the melt.
However, since the amount of carbon in the melt decreases at an end stage of the blowing, the generation of carbon monoxide rapidly diminishes and the reaction between the molten steel and slag rapidly goes down. Due to such decrease in decarburizing efficiency of oxygen, i.e. decrease in the proportion of oxygen which has been used to effect decarburization to the total amount of oxygen blown into the melt, the presence of excess oxygen is unavoidable resulting in oxidation of iron far beyond the equilibrium level. In addition, due to insufficient agitation of the molten steel and slag, there will be a temperature difference between them, resulting in a dephosphorizing reaction proceeding in an adverse direction. This is caused by less agitation of the molten metal. Therefore, it has been proposed to provide an oxygen converter with an electromagnetic agitator. It has also been proposed to add scrap iron to the melt at the last stage of blowing to generate a turbulence of the melt due to a temperature difference between the scrap and melt. However, these proposals have never been practiced because they require a high construction cost and their effect is not so large as expected.
Furthermore, it has been proposed to rotate or swing the oxygen blowing lance to provide additional agitation of molten metal and slag. But this promotes the agitation of slag, not of the molten steel.
In order to eliminate these prior art disadvantages, it has also been proposed to inject a blow of gas into a molten metal through the bottom while pure oxygen is blown onto the melt through a lance. Examples of the gases to be injected into the melt are limited to an inert gas such as argon and a neutral gas such as nitrogen. However, since argon is very expensive, and a relatively large amount must be blown into the melt in the bottom blowing so as to thoroughly agitate the melt, a sharp increase in cost is unavoidable. The introduction of pure nitrogen or a gas predominantly comprised of nitrogen, such as a compressed air will increase the nitrogen content of the melt. Thus, the blowing of nitrogen is not practical, either.
French Pat. No. 1,151,053 and U.S. Pat. No. 3,854,932 disclose the bottom blowing of various kinds of gases, including argon, steam, air carbon oxide, etc. However, U.S. Pat. No. 3,854,932, for example, is directed to the production of stainless steel, so that the main purpose is to suppress the oxidation of chromium. Thus, it is necessary to carry out the process of this invention under subatmospheric conditions. In addition, it treats these gases as being equivalent. Furthermore, since the French patent teaches the bottom blowing of a relatively large amount of gas into the melt, the process disclosed therein is less economical.
A recent development in this field is the "Q-Bop" process, in which instead of top-blowing of pure oxygen, the oxygen is blown into the molten metal through nozzles provided at the bottom of the converter. Since the "Q-Bop" process employs the bottom-blowing of pure oxygen gas instead of using the top-blowing thereof, it is necessary to blow gas such as propane for protecting the nozzles. Consequently, in this case, too, a relatively large amount of blowing gas must be injected into the molten metal. The "uniform mixing time" hereinafter described in detail is about 10 seconds.